U.S. Pat. No. 4,953,305 discloses an electrical compass system for use in a vehicle in which a magnetic sensor, such as a flux-gate sensor, is employed in connection with a microprocessor to provide a digital display of the vehicle heading. The system described in the '305 patent provides continuous automatic calibration of the compass to correct for changes in the detected magnetic field due to the vehicle's magnetism and changes thereto. The system described in the '305 patent also provides a system which, in effect, filters out magnetic aberrations caused by travel within an area where severe electromagnetic interference may temporarily occur.
In addition to the external magnetic disturbance that may affect the accuracy of display of compass headings, changes in the vehicle's magnetic field can also cause erroneous displays. One such problem is the type encountered when the compass sensor must be mounted in the instrument panel and the vehicle's HVAC system is activated. The fan motor draws significant current and, as the fan speed is varied drawing different amounts of current, the magnetic field caused by the current supplied to the fan motor can adversely affect the accuracy of the compass display. In order to correct for such a problem, the compensation system disclosed in U.S. Pat. No. 5,511,319 was developed. In this system, correction signals are provided to the compass circuit in response to sensed fan motor speed settings to continuously provide accurate heading information regardless of the fan speed setting.
Regardless of the mounting position of the compass sensor, the vehicle's magnetism can also be affected when, for example, a vehicle door is opened. Typically, when the vehicle is underway, the doors are always closed. When the vehicle stops, however, and the driver or passenger opens a door, the change in vehicle magnetism due to the movement of the door from a closed to an open position frequently will cause the compass heading to change even though the vehicle is stationary. U.S. Pat. No. 5,664,335 addresses this problem by using a door position detector circuit coupled to a microprocessor employed within the compass system. The compass system locks the compass display at the heading displayed just prior to opening of the vehicle door when the vehicle is not in operation.
Unfortunately, the systems that are disclosed in these patents have several drawbacks. U.S. Pat. No. 5,511,319 requires additional hardware, which increases overall system cost and complexity while reducing overall system reliability. U.S. Pat. No. 5,664,335 requires that the vehicle be stationary during the operation of the invention.
Accordingly, there is a need to correct for this magnetic noise during vehicle operation without requiring additional hardware. The system of the present invention provides an improved compass circuit and software that meets this objective.